GLACIATION OF THE YAKUTAT BAY REGION 229
Yakutat Bay. That this great expansion of the glaciers was due to some climatic change rather than solely to mountain uplift is indicated by the three following facts (1) the earlier genial climate, (2) the recent and present day recession (both 1 and 2 indicating that the climate of this region has changed), and (3) the widespread extension of glaciers ou the northwestern coast from Washington to the Aleutian Islands. While we cannot assert contemporaneity for the advance of glaciation throughout this region, the evidence points strongly toward the conclusion that the expanded stage of glaciation was contemporaneous throughout the region. It seems hardly probable that mountain uplift throughout a distance of some 2000 miles would alone suffice to give rise to such extensive glaciation, and since there has subsequently come an amelioration of the climate, while at a still earlier age there was an even milder climate than now, it is a reasonable inference to draw that the great expansion of glaciers was in response to actual climatic change.
It cannot be proved that the extensive Alaskan glaciation was contemporaneous with that of the Glacial Period in Europe and northeastern America, though it probably was is its inception. If this is so, then climatic change is without doubt the cause, for whatever the reason for the change, the spread of continental glaciers down into the temperate latitudes was a response to climatic change. Although we believe that the inception of the Alaskan glacier advance was contemporaneous with the spread of the continental glaciers in Europe and America, we are convinced that its termination was later than the period of recession of the continental glaciers. In a sense the Glacial Period is still in progress here, its influence being extended and prolonged by the lofty mountains, which have, in fact, partly or mainly reared themselves during the period of glaciation. Judging by the forest growth and by the extent of stream erosion since the recession of the ice from its most advanced position, we infer that the ice in its expanded stage lingered here until a few centuries ago.
One of our principal reasons for believing that the spread of glaciation in the Alaskan region dates back to the Glacial Period is that it has performed such vast work. Not only was a broad plain of glacial deposit built out into the Pacific beyond the mountain front, and an extensive submarine moraine built across the mouth of Yakutat Bay, but the topography was greatly altered by profound glacial erosion. This work must have required a long time. We cannot apply to this argument definite measurements from rate of ice erosion, but even the most extreme believer in the efficacy of ice erosion must agree that to lower a valley bottom from one to two thousand feet demands a great lapse of time. The main valleys have been greatly deepened; the lateral valleys have been deepened also, though to less extent; divides have been lowered and through valleys have been opened; the mountains have been sharpened; and, in fact, the topography has been completely altered in many of its most significant features by the glacial erosion resulting from the long flow of great ice streams. Thousands of years must be required for such vast work, and probably tens of thousands of years. It is by no means improbable that the beginning of extensive glaciation here dates back to the commencement of the Glacial Period, and that there was a long period of complex conditions of advance and recession with glacial and interglacial epochs. Of this, however, there is no proof. The period of glaciation was certainly many times longer than the period that has elapsed since the recession from the outermost stand.
When the glaciers were most advanced they literally flooded the mountain valleys,